When you're normally in a glycogen depleted state, carbs can most definitely add a few pounds very easily, almost instantly sometimes it seems.  Fortunately all it takes to lose that glycogen weight is a bit more carb restriction until your body depletes the reserves again, but doing that too often when your goal is weight loss can lead to spinning your wheels and not making any real progress.

I can jump 6-8lbs when carb loading over a weekend, then lose it all by Wednesday.

These might sound like silly questions, but are you getting enough sleep?  Are you still doing the long cardio workouts or have you backed off on those a bit?  Are your calories still really low?

While the weight gain sounds diet related, I'm wondering if the difficulty losing weight might be related to more than just diet...stress from sleep deprivation, over-training, and restricting calories can trick your body into thinking that times are tough, so it should store fat.

I get cravings from too much fruit too, by the way.  I still eat it, but save it for right around bedtime, and limit it to one serving.